Device, system and method for generating a photoplethysmographic image carrying vital sign information of a subject

ABSTRACT

The present invention relates to a device, system and a method for generating a photoplethysmographic image carrying vital sign information of a subject. To provide an increased validity and robustness against motion, in particular against ballistocardiographic motion, the proposed device comprises an input interface ( 30 ) for obtaining image data of a skin region of a subject in at least two different wavelength channels, said image data comprising two or more image frames acquired by detecting light transmitted through or reflected from the skin region over time, wherein said image data comprise wavelength-dependent reflection or transmission information in said at least two different wavelength channels, a combination unit ( 31 ) for combining, per pixel or group of pixels and per time instant, image data values of said at least two different wavelength channels to obtain a time-variant pulse signal per pixel or group of pixels, and an image generation unit ( 32 ) for generating a photoplethysmographic image from a property of the respective pulse signals in a time window including at least two image frames.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2017/050622, filed Jan. 13,2017, published as WO 2017/121834 on Jul. 20, 2017, which claims thebenefit of European Patent Application Number 16151494.8 filed Jan. 15,2016. These applications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a device, system and method forgenerating a photoplethysmographic image carrying vital sign informationof a subject, such as a person (e.g. a patient, elderly person, baby,etc.) or animal.

BACKGROUND OF THE INVENTION

Vital signs of a person, for example the heart rate (HR), therespiration rate (RR) or the arterial blood oxygen saturation, serve asindicators of the current state of a person and as powerful predictorsof serious medical events. For this reason, vital signs are extensivelymonitored in in-patient and out-patient care settings, at home or infurther health, leisure and fitness settings.

One way of measuring vital signs is plethysmography. Plethysmographygenerally refers to the measurement of volume changes of an organ or abody part and in particular to the detection of volume changes due to acardio-vascular pulse wave traveling through the body of a subject withevery heartbeat.

Photoplethysmography (PPG) is an optical measurement technique thatevaluates a time-variant change of light reflectance or transmission ofan area or volume of interest. PPG is based on the principle that bloodabsorbs light more than surrounding tissue, so variations in bloodvolume with every heart beat affect transmission or reflectancecorrespondingly. Besides information about the heart rate, a PPGwaveform can comprise information attributable to further physiologicalphenomena such as the respiration. By evaluating the transmittanceand/or reflectivity at different wavelengths (typically red andinfrared), the blood oxygen saturation can be determined.

Conventional pulse oximeters (also called contact PPG device herein) formeasuring the heart rate and the (arterial) blood oxygen saturation(also called SpO2) of a subject are attached to the skin of the subject,for instance to a fingertip, earlobe or forehead. Therefore, they arereferred to as ‘contact’ PPG devices. A typical pulse oximeter comprisesa red LED and an infrared LED as light sources and one photodiode fordetecting light that has been transmitted through patient tissue.Commercially available pulse oximeters quickly switch betweenmeasurements at a red and an infrared wavelength and thereby measure thetransmittance of the same area or volume of tissue at two differentwavelengths. This is referred to as time-division-multiplexing. Thetransmittance over time at each wavelength gives the PPG waveforms forred and infrared wavelengths. Although contact PPG is regarded as abasically non-invasive technique, contact PPG measurement is oftenexperienced as being unpleasant and obtrusive, since the pulse oximeteris directly attached to the subject and any cables limit the freedom tomove and might hinder a workflow. The same holds for contact sensors forrespiration measurements. Such contact sensors may sometimes bepractically impossible because of extremely sensitive skin (e.g. ofpatients with burns and preterm infants).

Recently, non-contact, remote PPG (rPPG) devices (also called camerarPPG device herein) for unobtrusive measurements have been introduced.Remote PPG utilizes light sources or, in general radiation sources,disposed remotely from the subject of interest. Similarly, also adetector, e.g., a camera or a photo detector, can be disposed remotelyfrom the subject of interest. Therefore, remote photoplethysmographicsystems and devices are considered unobtrusive and well suited formedical as well as non-medical everyday applications. However, remotePPG devices typically achieve a lower signal-to-noise ratio.

Verkruysse et al., “Remote plethysmographic imaging using ambientlight”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445demonstrates that photoplethysmographic signals can be measured remotelyusing ambient light and a conventional consumer level video camera,using red, green and blue color channels.

Using PPG technology, vital signs can be measured, which are revealed byminute light absorption changes in the skin caused by the pulsatingblood volume, i.e. by periodic color changes of the human skin inducedby the blood volume pulse. As this signal is very small and hidden inmuch larger variations due to illumination changes and motion, there isa general interest in improving the fundamentally low signal-to-noiseratio (SNR). There still are demanding situations, with severe motion,challenging environmental illumination conditions, or high requiredaccuracy of the application, where an improved robustness and accuracyof the vital sign measurement devices and methods is required,particularly for the more critical healthcare applications.

To achieve motion robustness, pulse-extraction methods profit from thecolor variations having an orientation in the normalized RGB color spacewhich differs from the orientation of the most common distortionsusually induced by motion. A known method for robust pulse signalextraction uses the known fixed orientation of the blood volume pulse inthe normalized RGB color space to eliminate the distortion signals.Further background is disclosed in G. de Haan and A. van Leest,“Improved motion robustness of remote-PPG by using the blood volumepulse signature”, Physiol. Meas. 35 1913, 2014, which describes that thedifferent absorption spectra of arterial blood and bloodless skin causethe variations to occur along a very specific vector in a normalizedRGB-space. The exact vector can be determined for a given light-spectrumand transfer-characteristics of the optical filters in the camera. It isshown that this “signature” can be used to design an rPPG algorithm witha much better motion robustness than the recent methods based on blindsource separation, and even better than chrominance-based methodspublished earlier.

A next challenge in camera-based vital sign monitoring is PPG imaging.Essentially, the camera-based approach is used to map the spatiallyvarying PPG amplitude and its derived vital signs (local SpO2, localperfusion, etc.). The hope is that this new technique will enable newdiagnostic means, e.g. for wound-healing, analysis of lesions (on-skinor internally after or during surgery), and cancer-detection (e.g.melanoma, but may be also oesophagus-cancer, colon-cancer, etc.).

A. A. Kamshilin, E. Nippolainen, I. S. Sidorov, P. V. Vasilev, N. P.Erofeev, N. P. Podolian, and R. V. Romashko, “A new look at the essenceof the imaging photoplethysmography,” Sci. Rep. 5 (2015) discloses asystem that builds PPG images from a monochrome camera (and mentionsoperating this system at 525 nm, which is close to 550 nm where thePPG-amplitude is strongest).

Markus Hülsbusch: “Ein bildgestütztes, funktionelles Verfahren zuroptoelektronischen Erfassung der Hautperfusion”, Dissertation,Technische Hochschule Aachen, 28 Jan. 2008, discloses an optoelectroniccamera based measurement concept for assessment of skin perfusion. Forthe detection and minimization of motion induced artifacts differentstrategies for movement compensation have been investigated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device, system anda method for generating a PPG image carrying vital sign information of asubject, which provide an increased validity and robustness againstmotion, in particular against ballistocardiographic (BCG) motion.

In a first aspect of the present invention, a device for generating aPPG image carrying vital sign information of a subject is presented, thedevice comprising:

an input interface for obtaining image data of a skin region of asubject in at least two different wavelength channels, said image datacomprising two or more image frames acquired by detecting lighttransmitted through or reflected from the skin region over time, whereinsaid image data comprise wavelength-dependent reflection or transmissioninformation in said at least two different wavelength channels,

a combination unit for combining, per pixel or group of pixels and pertime instant, image data values of said at least two differentwavelength channels to obtain a time-variant pulse signal per pixel orgroup of pixels, wherein said combination unit is configured to combinethe image data values of said at least two different wavelength channelsas a weighted combination of temporally normalized wavelength channelsor of the logarithm of wavelength channels, wherein the sum of theweights used for the combination is zero, and

an image generation unit for generating a photoplethysmographic imagefrom a property of the respective pulse signals in a time windowincluding at least two image frames.

In a further aspect of the present invention, a system for generating aPPG image carrying vital sign information of a subject is presented, thesystem comprising:

an imaging unit for acquiring image data of a skin region of a subjectin at least two different wavelength channels, said image datacomprising two or more image frames acquired by detecting lighttransmitted through or reflected from the skin region over time, whereinsaid image data comprise wavelength-dependent reflection or transmissioninformation in said at least two different wavelength channels, and

a device as disclosed herein for generating a photoplethysmographicimage carrying vital sign information of a subject from the acquiredimage data.

In yet further aspects of the present invention, there are provided acorresponding method, a computer program which comprises program codemeans for causing a computer to perform the steps of the methoddisclosed herein when said computer program is carried out on a computeras well as a non-transitory computer-readable recording medium thatstores therein a computer program product, which, when executed by aprocessor, causes the method disclosed herein to be performed.

Preferred embodiments of the invention are defined in the dependentclaims. It shall be understood that the claimed method, system, computerprogram and medium have similar and/or identical preferred embodimentsas the claimed device and as defined in the dependent claims.

Using a monochrome camera signal, as done in a known method, absorptionvariations in the skin due to varying blood volume cannot bedistinguished from variations due to motion. It is has been found thateven if care is taken to limit the influence of motion, the means(narrow band-pass filtering around heart rate, correlating with areference PPG signal) are ineffective, should the motion be synchronousto the pulse. Such cardiac-cycle synchronous motions are common nearbigger arteries, and even palpable, and can be suspected to play a roleat other places since the PPG-signal is very small. Particularly whenthe skin is non-uniformly illuminated, the angle between skin-normal andincident light is largest and the problem is exacerbated. The wrist'sskin at the vicinity of the brachial artery provides a clear example.

In an experiment the skin covering the brachial artery was covered byopaque ink, so only artifacts are acquired by the ink sensors. It can beobserved that the magnitude of ballistocardiographic motion artifacts iscomparable to the strongest PPG signals at the palm (0.005 AC/DC), andexceeds the amplitude of neighboring PPG signals at the wrist by afactor of five.

The present invention proposes an approach to significantly reduce theinfluence of motion on the PPG image. In particular, a mapping of two ormore different wavelength channels made such that the resulting pixelvalues are insensitive to motion, while still sensitive to PPGvariations. Biophysical (i.e. vital sign related) information, such as alocal PPG amplitude image or derived information is obtained bycombining the image data of different wavelength channels acquired overa time window of at least two image periods (i.e. covering at least twoimage periods) into at least one output PPG image, which carries thebiophysical information.

Hereby, the PPG image is generally generated from a property of therespective pulse signals in a time window including at least two imageframes. In particular, variations of the pulse signal are evaluated andreflected as image values of the PPG image in a spatial locationcorresponding to the pulse signal. The output image is thus, per timewindow, a single PPG image showing, in preferred embodiments, asproperty of the pulse signal, a function of the local perfusion (orlocal pulsatility, which may be understood to be the amplitude of the ACsignal part of the pulse signal (or PPG signal) normalized to the DClevel of the pulse signal (or PPG signal)), or the local phase of thepulse signal (the phase changes, e.g. due to travelling times of theblood, which may also depend on the size of the blood vessels imagedlocally), or the local amplitude of the pulse signal, or the standarddeviation of the pulse signal. The output image thus captures thespatial variation of the respective property of the individual pulsesignals. The scaling and bias (contrast and zero-level) of the spatialvariation can be varied if required (e.g. automatic scaling for maximumcontrast, or fixed scaling to know absolute pulsatility). The propertymay be mapped directly into an image, but may also be mapped using alinear or non-linear function, such as a gamma correction, subtractionof bias for improved contrast, squaring to show energy instead ofamplitude, using the energy instead of amplitude, or variance instead ofstandard deviation, etc.

The image data values of said at least two different wavelength channelsare combined as a weighted combination of temporally normalizedwavelength channels or of the logarithm of wavelength channels, whereinthe sum of the weights used for the combination is substantially zero.The present invention thus profits from the knowledge that motion, inparticular BCG motion, causes different relative strengths in thewavelength channels than the PPG signal. Hülsbusch, instead, makeschannel combinations (green and red) aiming at eliminating HF-ambientlight flickering and applies a weighted combination minimizing theenergy of the flickering. This cannot be used for eliminating BCG motionas this motion is pulse-frequent (same as the PPG-signal), and thus alsono proper weight can be determined according to Hülsbusch. In contrast,according to the present invention time-normalized wavelength channels(or the logarithmic version of the wavelength channels) are used so thatthe BCG motion disappears by choosing weights that add up to zero.

The computations may also be repeated on (non-overlapping or partiallyoverlapping) time windows and thus provide a video (i.e. an imagesequence) of the time-evolution of e.g. the local perfusion.

In an embodiment said combination unit is configured to combine theimage data values of said at least two different wavelength channels asa weighted sum, in particular of temporally normalized wavelengthchannels, such that the sum of the weights is substantially zero andwherein said image generation unit is configured to compute an amplitudemap and/or phase map of the temporal variations of said time-variantpulse signals as photoplethysmographic image.

In an embodiment said image generation unit is configured to compute theamplitude map as inner product of the respective pulse signal with areference signal and/or for computing the phase of the phase map withrespect to a reference signal. The device may further comprise acomputation unit for computing said reference signal using amotion-robust photoplethysmography signal extraction algorithm. Hereby,the reference signal may be computed from a skin area covered by aplurality of pixels in said image frames (i.e. from a large skin area)using a normalized blood volume pulse vector signature based method(i.e. a Pbv method), a chrominance based method (i.e. a CHROM method), ablind source separation method (i.e. a BSS method), a principalcomponent analysis (PCA) or an independent component analysis (ICA). Thereference signal may thus be obtained from a signal obtained byaveraging the signals of a selected group of pixels in the image.Alternatively, it may be obtained from a contact sensor mounted at thesubject.

Generally, a PPG signal results from variations of the blood volume inthe skin. Hence the variations give a characteristic pulsatility“signature” when viewed in different spectral components of thereflected/transmitted light. This signature is basically resulting asthe contrast (difference) of the absorption spectra of the blood andthat of the blood-less skin tissue. If the detector, e.g. a camera orsensor, has a discrete number of color channels, each sensing aparticular part of the light spectrum, then the relative pulsatilitiesin these channels can be arranged in a “signature vector”, also referredto as the “normalized blood-volume vector”, Pbv. It has been shown G. deHaan and A. van Leest, “Improved motion robustness of remote-PPG byusing the blood volume pulse signature”, Physiol. Meas. 35 1913, 2014,which is herein incorporated by reference, that if this signature vectoris known then a motion-robust pulse signal extraction on the basis ofthe color channels and the signature vector is possible. For the qualityof the pulse signal it is essential though that the signature iscorrect, as otherwise the known methods mixes noise into the outputpulse signal in order to achieve the prescribed correlation of the pulsevector with the normalized color channels as indicated by the signaturevector.

Details of the Pbv method and the use of the normalized blood volumevector (called “predetermined index element having a set orientationindicative of a reference physiological information”) have also beendescribed in US 2013/271591 A1, which details are also hereinincorporated by reference.

The computation unit may further be configured to compute said weightsfor weighting said at least two different wavelength channels. The samealgorithms and methods as used for computing said reference signal mayhereby be used. The weights may particularly be from a larger group ofpixels and not from the local pixel values themselves (since the SNR istoo low).

In still another embodiment said image generation unit is configured tofurther generate a motion map by computing a weighted difference betweenthe generated amplitude map and a single-channel amplitude map generatedfrom the image data of a single wavelength channel. Alternatively, thismotion map may be computed as a weighted sum of the different wavelengthchannels directly, using the “signature”-vector of motion ([1 1 1] for athree wavelength system), i.e. using the knowledge that the relativestrength of the motion signal is identical in all wavelength channels.The motion map may provide additional information useful for adiagnosis.

Furthermore, in an embodiment said image generation unit is configuredto use one or more of said amplitude map, phase map and motion map togroup image areas of the obtained image data showing motion artifactsbelow a predetermined motion artifact threshold or the smallest motionartifact and/or showing a ballistocardiographic motion above apredetermined ballistocardiographic motion threshold or the largestballistocardiographic motion and/or showing a photoplethysmographicinformation above a predetermined photoplethysmographic informationthreshold or the strongest photoplethysmographic information. Thegrouping may also be iterated in order to select the best camera sensors(i.e. image areas in the obtained image frames) to provide the referencesignal.

The image data are acquired by an imaging unit, in particular a camera,which acquires a temporal sequence of image frames of a skin region ofthe subject in at least two different wavelength channels. The imagingunit detects light, particularly in the wavelength interval between 400nm and 1200 nm, transmitted through or reflected from the skin region,wherein said image data comprise wavelength-dependent reflection ortransmission information in said at least two different wavelengthchannels.

Advantageously, said imaging unit comprises an optical sensing array, inparticular a two-dimensional image sensor, including a Bayer-patternfilter providing at least three wavelength channels, in particularcentered at approximately 450 nm, 550 nm and 650 nm or centered atapproximately 650 nm, 750 nm and 850 nm (which may have the advantagethat the penetration depth of the wavelengths is more comparable).However, it is also possible to use a separate 2D optical sensor foreach individual wavelength (each 2D optical sensor being equipped with adifferent filter, e.g. for 650 nm, 750 nm and 850 nm). In this case thesensors should be aligned (to sense the same skin area). This may bedone using optical means (e.g. a color-splitting prism), or usingelectronical means (e.g. image registration).

Generally, there exists a lot of freedom in choosing the wavelengths. Itis advantageous if the wavelengths correspond to spectral regions wherethe blood absorption is very different, although there may be reasonsthat prevent the most logical choice here, like preference for invisiblelight, limitations of the sensor, availability of efficient lightsources, etc.

Generally, the interaction of electromagnetic radiation, in particularlight, with biological tissue is complex and includes the (optical)processes of (multiple) scattering, backscattering, absorption,transmission and (diffuse) reflection. The term “reflect” as used in thecontext of the present invention is not to be construed as limited tospecular reflection but comprises the afore-mentioned types ofinteraction of electromagnetic radiation, in particular light, withtissue and any combinations thereof.

For obtaining a vital sign information signal of the subject the datasignals of skin pixel areas within the skin area are evaluated. Here, a“skin pixel area” means an area comprising one skin pixel or a group ofadjacent skin pixels, i.e. a data signal may be derived for a singlepixel or a group of skin pixels.

The system may further comprise an illumination unit, such as a lightsource positioned remotely from the tissue, for illuminating the skinregion of the subject with light in said at least two differentwavelength channels. This further improves the acquisition of the imagedata and the quality of the obtained PPG image.

Preferably, said illumination unit is configured to emit modulated lighthaving a modulation frequency outside the frequency band of thesubject's pulse, in particular above 200 BPM. For instance, in anembodiment said illumination unit is configured to emit amplitudemodulated light using a modulation signal, in particular 1+β·sin(½πf)with a small modulation factor β in the range of 0.001<β<0.1. Instead ofmodulating the illumination unit it is equally possible to apply thismodulation to all individual wavelength channels using constantillumination, which provides a comparable effect.

In another embodiment the illumination unit is configured to emitpolarized light and the imaging unit comprises a polarizer. This reducesthe sensitivity for specular reflections on the skin.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter. Inthe following drawings:

FIG. 1 shows a schematic diagram of a first embodiment of a systemaccording to the present invention,

FIG. 2 shows a diagram of the absorption spectrum of oxygenated andnon-oxygenated blood,

FIG. 3 shows a schematic diagram of a first embodiment of a deviceaccording to the present invention,

FIG. 4 shows a schematic diagram of a second embodiment of a systemaccording to the present invention, and

FIG. 5 shows a schematic diagram of a second embodiment of a deviceaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic diagram of a system 10 according to the presentinvention including a device 12 for generating a photoplethysmographicimage (PPG image) carrying vital sign information (in particular a vitalsign information signal) of a subject of a subject 14 from detectedelectromagnetic radiation transmitted through or reflected from asubject. The subject 14, in this example a patient, lies in a bed 16,e.g. in a hospital or other healthcare facility, but may also be aneonate or premature infant, e.g. lying in an incubator, or person athome or in a different environment, such as an athlete doing sports. Theregion of interest may be skin, but may also be an internal organ, inwhich case the images may be recorded in an operation theatre duringsurgery. In the following explanation a skin area will be used as anexample for the region of interest, but the explanation shall be equallyapply to an internal organ as region of interest.

For the following explanation, the vital sign information to bedetermined may be pulsatility or blood perfusion, but other kinds ofvital sign information may also be obtained and depicted in a PPG image.

There exist different embodiments for an imaging unit (or detector) fordetecting electromagnetic radiation transmitted through or reflectedfrom a subject, which may alternatively (which is preferred) or togetherbe used. In the embodiment of the system 10 two different embodiments ofthe imaging unit are shown and will be explained below. Both embodimentsof the imaging are configured for acquiring image data of a skin regionof a subject in at least two different wavelength channels, said imagedata comprising two or more image frames acquired by detecting lighttransmitted through or reflected from the skin region over time, whereinsaid image data comprise wavelength-dependent reflection or transmissioninformation in said at least two different wavelength channels.

In one embodiment the imaging unit comprises a camera 18 (also referredto as camera-based or remote PPG sensor) including a suitablephotosensor for (remotely and unobtrusively) capturing image frames ofthe subject 14, in particular for acquiring a sequence of image framesof the subject 14 over time, from which photoplethysmography signals canbe derived. The image frames captured by the camera 18 may particularlycorrespond to a video sequence captured by means of an analog or digitalphotosensor, e.g. in a (digital) camera. Such a camera 18 usuallyincludes a photosensor, such as a CMOS or CCD sensor, which may alsooperate in a specific spectral range (visible, IR) or provideinformation for different spectral ranges. The camera 18 may provide ananalog or digital signal. The image frames include a plurality of imagepixels having associated pixel values. Particularly, the image framesinclude pixels representing light intensity values captured withdifferent photosensitive elements of a photosensor. These photosensitiveelements may be sensitive in a specific spectral range (i.e.representing a specific color or a weighted sum of wavelengths). Theimage frames include at least some image pixels being representative ofa skin portion of the subject. Thereby, an image pixel may correspond toone photosensitive element of a photo-detector and its (analog ordigital) output or may be determined based on a combination (e.g.through binning, or spatial filtering) of a plurality of thephotosensitive elements.

In another embodiment the imaging unit comprises one or more opticalphotoplethysmography sensor(s) 19 (also referred to as contact PPGsensor(s)) configured for being mounted to a skin portion of the subject14 for acquiring photoplethysmography signals. The PPG sensor(s) 19 maye.g. be designed in the form of a patch attached to a subject's foreheadfor measuring the blood oxygen saturation or a heart rate sensor formeasuring the heart rate, just to name a few of all the possibleembodiments.

When using a camera 18 the system 10 may further optionally comprise anillumination unit 22 (also called illumination source or light source orelectromagnetic radiator), such as a lamp or LED, forilluminating/irradiating a region of interest 24, such as the skin ofthe patient's face (e.g. part of the cheek or forehead), with light, forinstance in a predetermined wavelength range or ranges (e.g. in the red,green and/or infrared wavelength range(s)). The light reflected fromsaid region of interest 24 in response to said illumination is detectedby the camera 18. In another embodiment no dedicated light source isprovided, but ambient light is used for illumination of the subject 14.From the reflected light only light in a desired wavelength ranges (e.g.green and red or infrared light, or light in a sufficiently largewavelength range covering at least two wavelength channels) may bedetected and/or evaluated.

The device 12 is further connected to an interface 20 for displaying thedetermined information and/or for providing medical personnel with aninterface to change settings of the device 12, the camera 18, the PPGsensor(s) 19, the light source 22 and/or any other parameter of thesystem 10. Such an interface 20 may comprise different displays,buttons, touchscreens, keyboards or other human machine interface means.

A system 10 as illustrated in FIG. 1 may, e.g., be located in ahospital, healthcare facility, elderly care facility or the like. Apartfrom the monitoring of patients, the present invention may also beapplied in other fields such as neonate monitoring, general surveillanceapplications, security monitoring or so-called live style environments,such as fitness equipment, a wearable, a handheld device like asmartphone, or the like. The uni- or bidirectional communication betweenthe device 12, the camera 18, the PPG sensor(s) 19 and the interface 20may work via a wireless or wired communication interface. Otherembodiments of the present invention may include a device 12, which isnot provided stand-alone, but integrated into the camera 18 or theinterface 20.

There exist several known methods to obtain a pulse signal S from(normalized) detection signals C_(n), said methods being referred to asICA, PCA, Pbv, CHROM, and ICA/PCA guided by Pbv/CHROM, which have alsobeen described in the above cited paper of de Haan and van Leest. Thesemethods can be interpreted as providing the pulse signal S as a mixtureof different wavelength channels, e.g. red, green and blue signals froma color video camera, but they differ in the way to determine theoptimal weighting scheme. In these methods the resulting weights areaimed at a mixture in which the distortions disappear, i.e. the“weighting vector” is substantially orthogonal to the main distortionsusually caused by subject motion and/or illumination variations.

In the following some basic considerations with respect to the Pbvmethod shall be briefly explained.

The beating of the heart causes pressure variations in the arteries asthe heart pumps blood against the resistance of the vascular bed. Sincethe arteries are elastic, their diameter changes in sync with thepressure variations. These diameter changes occur even in the smallervessels of the skin, where the blood volume variations cause a changingabsorption of the light.

The unit length normalized blood volume pulse vector (also calledsignature vector) is defined as Pbv, providing the relative PPG-strengthin the red, green and blue camera signal. To quantify the expectations,the responses H_(red)(w), H_(green)(w) and H_(blue)(w) of the red, greenand blue channel, respectively, were measured as a function of thewavelength w, of a global-shutter color CCD cameral, the skinreflectance of a subject, ρ_(s)(w), and used an absolute PPG-amplitudecurve PPG(w). From these curves, shown e.g. in FIG. 2 of the above citedpaper of de Haan and van Leest, the blood volume pulse vector P_(bv) iscomputed as:

${\overset{\rightarrow}{\hat{P}}}_{bv}^{T} = \begin{bmatrix}\frac{\int\limits_{w = 400}^{700}{{H_{red}(w)}{I(w)}{{PPG}(w)}\mspace{14mu}{dw}}}{\int\limits_{w = 400}^{700}{{H_{red}(w)}{I(w)}{\rho_{a}(w)}\mspace{14mu}{dw}}} \\\frac{\int\limits_{w = 400}^{700}{{H_{green}(w)}{I(w)}{{PPG}(w)}\mspace{14mu}{dw}}}{\int\limits_{w = 400}^{700}{{H_{green}(w)}{I(w)}{\rho_{a}(w)}\mspace{14mu}{dw}}} \\\frac{\int\limits_{w = 400}^{700}{{H_{blue}(w)}{I(w)}{{PPG}(w)}\mspace{14mu}{dw}}}{\int\limits_{w = 400}^{700}{{H_{blue}(w)}{I(w)}{\rho_{a}(w)}\mspace{14mu}{dw}}}\end{bmatrix}$which, using a white, halogen illumination spectrum I(w), leads to anormalized Pbv=[0.27, 0.80, 0.54]. When using a more noisy curve theresult may be Pbv=[0.29, 0.81, 0.50].

The blood volume pulse predicted by the used model correspondsreasonably well to an experimentally measured normalized blood volumepulse vector, Pbv=[0.33, 0.77, 0.53] found after averaging measurementson a number of subjects under white illumination conditions. Given thisresult, it was concluded that the observed PPG-amplitude, particularlyin the red, and to a smaller extent in the blue camera channel, can belargely explained by the crosstalk from wavelengths in the intervalbetween 500 and 600 nm. The precise blood volume pulse vector depends onthe color filters of the camera, the spectrum of the light and theskin-reflectance, as the model shows. In practice the vector turns outto be remarkably stable though given a set of wavelength channels (thevector will be different in the infrared compared to RGB-based vector).

It has further been found that the relative reflectance of the skin, inthe red, green and blue channel under white illumination does not dependmuch on the skin-type. This is likely because the absorption spectra ofthe blood-free skin is dominated by the melanin absorption. Although ahigher melanin concentration can increase the absolute absorptionconsiderably, the relative absorption in the different wavelengthsremains the same. This implies an increase of melanin darkens the skin,but hardly changes the normalized color of the skin. Consequently, alsothe normalized blood volume pulse P_(bv) is quite stable under whiteillumination. In the infrared wavelengths the influence of melanin isfurther reduced as its maximum absorption occurs for short wavelengths(UV-light) and decreases for longer wavelengths.

The stable character of Pbv can be used to distinguish color variationscaused by blood volume change from variations due to alternative causes.The resulting pulse signal S using known methods can be written as alinear combination (representing one of several possible ways of“mixing”) of the individual DC-free normalized color channels:S=W C _(n)with WW^(T)=1 and where each of the three rows of the 3×N matrix C_(n)contains N samples of the DC-free normalized red, green and blue channelsignals R_(n), G_(n) and B_(n), respectively, i.e.:

${{\overset{\rightarrow}{R}}_{n} = {{\frac{1}{\mu\left( \overset{\rightarrow\;}{R} \right)}\overset{\rightarrow}{R}} - 1}},{{\overset{\rightarrow}{G}}_{n} = {{\frac{1}{\mu\left( \overset{\rightarrow}{G} \right)}\overset{\rightarrow}{G}} - 1}},{{\overset{\rightarrow}{B}}_{n} = {{\frac{1}{\mu\left( \overset{\rightarrow}{B} \right)}\overset{\rightarrow}{B}} - 1.}}$

Here the operator μ corresponds to the mean. Key difference between thedifferent methods is in the calculation of the weighting vector W. Inone method, the noise and the PPG signal may be separated into twoindependent signals built as a linear combination of two color channels.One combination approximated a clean PPG signal, the other containednoise due to motion. As an optimization criterion the energy in thepulse signal may be minimized. In another method a linear combination ofthe three color channels may be used to obtain the pulse signal. Instill further methods, the ICA or the PCA may be used to find thislinear combination. Since it is a priori unknown which weighted colorsignal is the pulse signal all of them used the periodic nature of thepulse signal as the selection criterion.

The Pbv method generally obtains the mixing coefficients using the bloodvolume pulse vector as basically described in US 2013/271591 A1 and theabove cited paper of de Haan and van Leest. The best results areobtained if the band-passed filtered versions of R_(n), G_(n) and B_(n)are used. According to this method the known direction of Pbv is used todiscriminate between the pulse signal and distortions. This not onlyremoves the assumption (of earlier methods) that the pulse is the onlyperiodic component in the video, but also eliminates assumptions on theorientation of the distortion signals. To this end, it is assumed asbefore that the pulse signal is built as a linear combination ofnormalized color signals. Since it is known that the relative amplitudeof the pulse signal in the red, green and blue channel is given by Pbv,the weights, W_(PBV), are searched that give a pulse signal S, for whichthe correlation with the color channels R_(n), G_(n), and B_(n) equalsPbv{right arrow over (S)}C _(n) ^(T) =k{right arrow over (P)} _(bv) ⇔{rightarrow over (W)} _(PBV) C _(n) C _(n) ^(T) =k{right arrow over (P)}_(bv),  (1)and consequently the weights determining the mixing are determined by{right arrow over (W)} _(PBV) =k{right arrow over (P)} _(bv) Q ⁻¹ withQ=C _(n) C _(n) ^(T),  (2)and the scalar k is determined such that W_(PBV) has unit length. It isconcluded that the characteristic wavelength dependency of the PPGsignal, as reflected in the normalized blood volume pulse, Pbv, can beused to estimate the pulse signal from the time-sequential RGB pixeldata averaged over the skin area. This algorithm is referred to as theP_(bv) method.

Hence, as explained above, a pulse signal results as a weighted sum ofthe at least two detection signals C_(n). Since all detection signalsC_(n) contain the pulse and different levels of (common) noise, theweighting (of the detection signals to obtain the pulse signal) can leadto a pure noise-free pulse. This is why ICA and PCA can be used toseparate noise and pulse. According to the present invention this isdone differently.

FIG. 2 shows a diagram of the absorption spectra of blood for oxygenatedblood (SpO2=100%) and non-oxygenated blood (SpO2=60%). As can be seen,the absorption spectrum of blood depends on the oxygen saturation,particularly in the wavelengths around 650 nm. It is clear from FIG. 2though that the absorption in the green wavelength range (around 550 nm)and blue wavelength range (around 450 nm) is much higher.

FIG. 3 shows a schematic illustration of an embodiment of the device 12according to the present invention. The device 12 comprises an inputinterface 30 for obtaining image data of a skin region of a subject inat least two different wavelength channels, said image data comprisingtwo or more image frames acquired by detecting light transmitted throughor reflected from the skin region over time, wherein said image datacomprise wavelength-dependent reflection or transmission information insaid at least two different wavelength channels. A combination unit 31combines, per pixel or group of pixels and per time instant, image datavalues of said at least two different wavelength channels to obtain atime-variant pulse signal per pixel or group of pixels. An imagegeneration unit 32 generates a photoplethysmographic image from aproperty of the respective pulse signals in a time window including atleast two image frames. The device 12 may e.g. be implemented in theform of a processor or computer, i.e. in software and/or hardware.

Preferably, the combination unit 31 combines the image data from thedifferent wavelength channels as a weighted sum of temporally normalizedchannels in a temporal window, such that the sum of the weights issubstantially zero, and image generation unit 32 computes an amplitude-and/or phase-map of the time-variations for each (group of) pixels asthe output image, i.e. of the temporal variations of said time-variantpulse signals as photoplethysmographic image. Hereby, the amplitude mapmay be computed as inner product of the respective pulse signal with areference signal and/the phase of the phase map may be computed withrespect to a reference signal, which may be obtained by averaging all,or a subset of, the signals and normalizing the result). Similarly, thephase of the phase map may be computed with respect to a referencesignal, which may be obtained from a contact sensor or from a signalobtained by averaging the signals of a selected group of pixels in theimage.

The device 12 thus is not merely applying known motion robust PPGextraction, as described in many publications, per group of pixels. Thiswould not work out well, since the thus-obtained channel weights wouldbe too noisy, which would lead to inaccurate PPG amplitudes. The reasonsare the limited number of pixels that are averaged per sensor, and thevery small motion distortions in the typically immobilized body part. Tosolve this issue, multiplicative noise or a modulated light source maybe applied in preferred embodiments (both are options to emulatestronger motion) to obtain stable weights suitable to suppress motion(using e.g. known motion robust PPG extraction methods on a relativelylarge skin area). These resulting weights are then successively used onindividual pixel (groups) for channel mapping.

In an embodiment said image generation unit 32 is configured todetermine, per pulse signal, a function of one or more of thepulsatility, amplitude, phase and standard deviation in said time windowand to use the determined function of pulsatility, amplitude, phaseand/or standard deviation as image data value of the PPG image at thespatial location corresponding to the respective pulse signal. Hence,for each image pixel (spation location) of the PPG image a correspondingpulse signal for said location is analyzed within a time window, inparticular a selected property is analyzed, which may be pulsatility,amplitude, phase and/standard deviation. The result of this analysis ora function of the analyzed property, e.g. the pulsatility itself or anaverage of the amplitude, is then used as the pixel value at thisparticular pixel of the PPG image.

Optionally, the device 12 further comprises a computation unit 33 forcomputing weights for determining the above mentioned weighted sumsand/or for computing said above mentioned reference signal using amotion-robust photoplethysmography signal extraction algorithm. Theweights and/or said reference signal may hereby be computed from alarger skin area covered by a plurality of pixels in said image framesusing one or the above mentioned normalized blood volume pulse vectorsignature based method (Pbv method), a chrominance based method (CHROM),a blind source separation (BSS) method, a principal component analysis(PCA) or an independent component analysis (ICA).

In more detail, a reference signal may be obtained from a region ofinterest (ROI) within the recorded image frames, with a high ratiobetween signal and noise-plus interference (e.g., minimizing sensornoise by selecting a large ROI, reducing motions physically byimmobilization, uniform lighting conditions during data acquisition,etc.). The pixels of this reference ROI are combined (e.g. averaged),the result being a modulated stream per each camera channel. Thesestreams are then preferably fed to the CHROM or Pbv method. Both thesemethods produce weights given to the individual wavelength channels toobtain a motion robust output PPG signal. The resulting weights ofeither method can consequently be used for the channel-mapping of allpixels or groups of pixels.

As output image a motion map may be determined, which results as aweighted difference between the amplitude map resulting from channelmapping and an amplitude map from a single wavelength channel.Alternatively, a weighted sum of the different wavelength channels,reflecting the motion map, may be computed directly.

Further, one or more of said amplitude map, phase map and motion map maybe used to group image areas of the obtained image data showing motionartifacts below a predetermined motion artifact threshold or thesmallest motion artifact and/or showing a ballistocardiographic motionabove a predetermined ballistocardiographic motion threshold or thelargest ballistocardiographic motion and/or showing aphotoplethysmographic information above a predeterminedphotoplethysmographic information threshold or the strongestphotoplethysmographic information. Hence, camera sensors may be grouped(e.g. into a group with low-level motion artifacts, or a group with highlevel of BCG-motion, etc.). The signals from the grouped sensors may becombined into single signals for further analysis (e.g. waveform ofPPG/BCG-signals). The grouping may also be iterated in order to selectthe best camera-sensors to provide a high quality reference signal,prior to deriving the physiological information. Examples of skin siteswhere BCG-motion is expected to be strong are e.g., carotid artery atthe neck, brachial artery at the wrist, femoral artery and poplitealartery at the lower limbs, etc. Further examples are high-contrast areas(e.g. wrinkles, hairs) and edges (boundaries of an image part, e.g.space between two fingers), and body parts that are hard to immobilize,like eyes.

Still further, a series of output images obtained from time-shifted timewindows is computed and shown sequentially. The time windows ofsuccessive output images may or may not be partially overlapping.

FIG. 4 shows a schematic diagram of a second embodiment of a system 11according to the present invention. In this embodiment the camera 18comprises an optical sensing array, in particular a two-dimensionalimage sensor, including a Bayer-pattern filter 180 providing at leastthree wavelength channels, in particular centered at approximately 450nm, 550 nm and 650 nm or centered at approximately 650 nm, 750 nm and850 nm. In another embodiment (not shown) separate two-dimensional imagesensors may be used, one for each respective wavelength channel.

The illumination unit 22 preferably emits light in a wavelength rangecovering the three wavelength channels, i.e. in a range between 400 nmand 1200 nm. The illumination unit may even be configured to emit lightat the same wavelength channels as mentioned above, i.e. centered atapproximately 450 nm, 550 nm and 650 nm or centered at approximately 650nm, 750 nm and 850 nm.

The illumination unit 22 may further be configured or controlled to emitmodulated light having a modulation frequency outside the frequency bandof the subject's pulse, in particular above 200 BPM. Particularly,amplitude modulated light may be emitted using a modulation signal, inparticular 1+β·sin(½πf) with a small modulation factor β in the range of0.001<β<0.1.

Still further, the illumination unit may emit polarized light using apolarizer 23, and the imaging unit 18 may also comprise a correspondingpolarizer 181 to reduce the sensitivity for specular reflections on theskin. To this end, the polarizers 23 and 181 may be orthogonal, suchthat primarily the scattered light that returns after penetrating theskin reaches the camera, while the specularly reflected light, which hasthe same polarization as the light-source, is substantially blocked.

FIG. 5 shows a schematic diagram of a second embodiment of a device 12′according to the present invention. In a preprocessing unit 40 imageframes are registered with respect to a central image frame of anacquired video sequence. A Horn-Schunck algorithm may be used to ensurestabilization against small movements, even at wrinkles and contours ofthe skin. The resulting image frames are subsequently denoised byGaussian blurring and reduced by a factor of 5. Each pixel in theobtained images are referred to as a sensor element. Finally, skinsensors are segmented from background (dark, textureless) by colorthresholding, and a large reference skin ROI (ROI Skin) is (preferablymanually) demarcated, e.g. at the palm.

To obtain a reference remote PPG (rPPG) signal in a reference signalgeneration unit 41, a raw RGB stream is extracted from a user-definedROI, e.g. at the palm. In a mapping unit 42 the reference signal isfirst low pass filtered to extract its “DC” component (e.g. using a 9thorder Butterworth filter; cutoff frequency, 20 BPM) and normalized asAC/DC. The signals are then processed in strides of 128 samples(corresponding to about 10 cardiac cycles) with an overlapping factor of50%. Each stride is detrended, multiplied with a Hanning window andfiltered in the frequency domain by selection of heart ratecomponent(s). The heart rate measurements (i.e. the instantaneous pulserate) is obtained from pulse oximetry (cPPG), e.g. through peakdetection in a detection unit 49. The heart rate measurements obtainedby contact PPG (cPPG) are used to select either one or more harmonics ofthe pulse signal. This is referred to as adaptive bandpass filtering(ABPF) in ABPF unit 43. The signal is then, in transformation unit 44,Hilbert-transformed and normalized to unit norm asΣRe[{tilde over (x)} _(ref)]{tilde over (x)} _(ref)=1.

The initial processing stages in a local signal generation unit 45 forobtaining a local rPPG amplitude and phase in each sensor element aresimilar to those of the reference remote PPG signal in unit 41, i.e.,the raw RGB streams in each column m and line n of the sensor array are,in a mapping unit 46, normalized to AC/DC, mapped according to CHROM orPBV and, in an ABPF unit 47, adaptively bandpass filtered. The value ofthe PPG image at (m, n), obtained in a correlation unit 48, is thenormalized inner-product between X_(Skin) and s_(m,n), i.e.PPGI_(m,n)=√{square root over (2/L)}Σ_(l=1) ^(L) s _(m,n)(l){tilde over(x)} _(Ref)(l).

In the described embodiments, so far, the “PBV-method” as described e.g.in the above cited paper of G. de Haan and A. van Leest, “Improvedmotion robustness of remote-PPG by using the blood volume pulsesignature”, is used as a basis for the computations. In furtherembodiments it is possible to use alternatives to W_(PBV). It is equallypossible to use any of the other methods mentioned in this paper tocompute the weights used to combine the color channels to a vital signsignal with minimal distortions. Particularly, a good solution alsoresults when using the chrominance based method, “CHRO”, but also the“guided BSS-based methods” and even the older BSS-based methods, usingperiodicity of the pulse signal for component selection, provide viableoptions. Generally, the weights are calculated from the color signalsfiltered to include at least the pulse signal variations, in case onlytwo wavelength channels are used, e.g. green and red, the difference ofthe normalized green and red also provides a viable option, which canmathematically be shown to approximate the ratio of green and redchannels (motion has same strength in both channels and falls out,Pulsatility is different and remains. In this case fixed weights (1 and−1: Gn−Rn=Gn/Rn) may be used).

The present invention considers, for the first time, cancellation ofballistocardiographic artifacts in remote PPG images by means of signalprocessing. As this kind of motion is synchronous with the cardiacsignal, common strategies to enhance the motion robustness areineffective against this interfering source, and risks are thatartifacts are confounded with actual PPG signals. According to thepresent invention, a step further has been taken to improving validityof PPG images by demonstrating that two known motion-robust channelmapping algorithms previously reported for heart rate detection in theremote-PPG literature, namely CHROM and PBV, can be extended to PPGimaging. These offer the advantage of eliminating motion sources inremote PPG sensor elements, irrespective of whether they arecardiac-related or from other sources, with the added benefit ofcompensating artifacts resulting from non-orthogonal illumination incurved skin surfaces. Performance gains resulting from motion robustchannel-mapping are cumulative with additional practices or signalprocessing approaches aiming at improved PPG-image formation.

The joint representation of the normalized Fourier coefficients forremote-PPG signals (a surrogate of blood-volume changes at the capillarybed) and BCG-artifacts (a surrogate of arterial motion) further confirmsthat these signals are slightly different and must not be confused.Using the same recordings, the feasibility of BCG-artifact cancellationby mapping normalized data from the PPG sensor elements prior to PPGimage formation has been confirmed. Using the CHROM- or PBV-basedimaging frameworks, it was observed that the PPG amplitude in the palmregion is stronger than at the wrist, for all subjects. In the correctedphase images, differences in the order of 20-30 degrees were observedbetween the center of the palm and wrist, and the periphery of the palm;these are largely independent of illumination conditions, i.e., lateralor homogeneous illumination show similar results. Pulse-induced skinmotion patterns are most prominent under non-uniform lightingconditions, though, to a minor extent, BCG-artifacts also occur underuniform lighting conditions, at high spatial frequency sites, as areedges, wrinkles, and even the texture of the skin.

In this investigation, both algorithms performed comparably. Inpractical scenarios, the preference from one method over the othershould depend upon the relative ease of estimating the blood-volumepulse vector, PBV or the trust in the assumed “standard skin-tonevector”. Performance benefits resulting from the inclusion of twoadditional harmonics of the pulse-rate frequency were assessed, and itwas found that it resulted in just marginal improvements in detail andNRMS performance of the resulting PPG images.

The above described embodiments have mainly been explained with respectto contactless sensors. Generally, the same methods can also be used forcontact sensors. By way of example, the present invention can be appliedin the field of health care, e.g. unobtrusive remote patient monitoring,general surveillances, security monitoring and so-called lifestyleenvironments, such as fitness equipment, or the like. Applications mayinclude a finger oximeter or unobtrusive monitoring. Particularly fornew diagnostic means, e.g. for wound-healing, analysis of lesions(on-skin or internally after or during surgery), and cancer-detection(on skin, e.g. melanoma, but may be also in-body, e.g.oesophagus-cancer, colon-cancer, etc.).

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

A computer program may be stored/distributed on a suitable medium, suchas an optical storage medium or a solid-state medium supplied togetherwith or as part of other hardware, but may also be distributed in otherforms, such as via the Internet or other wired or wirelesstelecommunication systems.

Any reference signs in the claims should not be construed as limitingthe scope.

The invention claimed is:
 1. A device for generating aphotoplethysmographic image carrying vital sign information of asubject, the device comprising: an input interface circuit that obtainsimage data of a skin region of a subject in at least three differentwavelength channels, wherein the image data comprises two or more imageframes acquired by detecting light transmitted through or reflected fromthe skin region over time, wherein the image data compriseswavelength-dependent reflection or transmission information in the atleast three different wavelength channels, a combination circuit thatcombines, per group of one or more pixels of the image frames and pertime instant, image data values of the at least three differentwavelength channels to obtain a time-variant pulse signal per group,wherein the combination circuit is configured to combine the image datavalues of the at least three different wavelength channels as a weightedcombination of one of: temporally normalized wavelength channels, andlogarithm of wavelength channels, wherein the sum of the weights usedfor the combination is zero, and an image generation circuit thatgenerates a photoplethysmographic image from a property of therespective pulse signals in a time window including at least two imageframes.
 2. The device as claimed in claim 1, wherein the imagegeneration circuit is configured to determine, per pulse signal, afunction of one or more of the pulsatility, amplitude, phase, andstandard deviation in the time window and wherein the image generationcircuit is configured to use the determined function of the one or morepulsatility, amplitude, phase, and standard deviation as image datavalue of the photoplethysmographic image at the spatial locationcorresponding to the respective pulse signal.
 3. The device as claimedin claim 1, wherein the image generation circuit is configured tocompute at least one of an amplitude map and a phase map of the temporalvariations of the time-variant pulse signals as thephotoplethysmographic image.
 4. The device as claimed in claim 3,wherein the image generation circuit is configured to compute at leastone of: the amplitude map as an inner product of the respective pulsesignal with a reference signal, and the phase of the phase map withrespect to a reference signal.
 5. The device as claimed in claim 3,wherein the computation circuit computes at least one of: weights fordetermining the weighted sums and the reference signal using amotion-robust photoplethysmography signal extraction algorithm.
 6. Thedevice as claimed in claim 5, wherein the computation circuit isconfigured to compute the at least one of: the weights and the referencesignal from a skin area covered by a plurality of pixels in the imageframes using at least one of: a normalized blood volume pulse vectorsignature based method, a chrominance based method, a blind sourceseparation method, a principal component analysis, and an independentcomponent analysis.
 7. The device as claimed in claim 3, wherein theimage generation circuit is configured to use one or more of theamplitude map, the phase map and the motion map to group image areas ofthe obtained image data showing at least one of: motion artifacts belowa predetermined motion artifact threshold, motion artifacts below asmallest motion artifact, a ballistocardiographic motion above apredetermined ballistocardiographic motion threshold, aballistocardiographic motion above a largest ballistocardiographicmotion, a photoplethysmographic information above a predeterminedphotoplethysmographic information threshold, and a photoplethysmographicinformation above a strongest photoplethysmographic information.
 8. Adevice for generating a photoplethysmographic image carrying vital signinformation of a subject, the device comprising: an input interfacecircuit that obtains image data of a skin region of a subject in atleast two different wavelength channels, the image data comprising twoor more image frames acquired by detecting light transmitted through orreflected from the skin region over time, wherein the image datacomprise wavelength-dependent reflection or transmission information inthe at least two different wavelength channels, a combination circuitthat combines, per a group of one or more pixels of the image frames andper time instant, image data values of the at least two differentwavelength channels to obtain a time-variant pulse signal per group,wherein the combination circuit is configured to combine the image datavalues of the at least two different wavelength channels as a weightedcombination of one of: temporally normalized wavelength channels, andlogarithm of wavelength channels, wherein the sum of the weights usedfor the combination is zero, and an image generation circuit thatgenerates a photoplethysmographic image from a property of therespective pulse signals in a time window including at least two imageframes; wherein the image generation circuit is configured to compute atleast one of an amplitude map and phase map of the temporal variationsof the time-variant pulse signals as the photoplethysmographic image,wherein the image generation circuit is configured to generate a motionmap by computing at least one of: a weighted difference between thegenerated amplitude map and a single-channel amplitude map generatedfrom the image data of a single wavelength channel, and a weighted sumof the image data values of the different wavelength channels.
 9. Asystem for generating a photoplethysmographic image carrying vital signinformation of a subject, the system comprising: the device as claimedin claim 1, and an imaging circuit that acquires the image data.
 10. Thesystem as claimed in claim 9, wherein the imaging circuit comprises anoptical sensing array, wherein the optical sensing array comprises atleast one of: a Bayer-pattern filter providing at the least threewavelength channels, wherein the at least three wavelength channels arenominally centered at one of: 450 nm, 550 nm and 650 nm, respectively,and 650 nm, 750 nm and 850 nm, respectively; and separatetwo-dimensional image sensors for each of the respective at least threewavelength channels.
 11. The system as claimed in claim 9, furthercomprising an illumination circuit that illuminates the skin region ofthe subject with light in the at least three different wavelengthchannels.
 12. The system as claimed in claim 11, wherein theillumination circuit is configured to emit modulated light having amodulation frequency outside the frequency band of the subject's pulse.13. The system as claimed in claim 12, wherein the illumination circuitis configured to emit amplitude modulated light using a modulationsignal of 1+β·sin(½πf), wherein 0.001<β<0.1.
 14. A method for generatinga photoplethysmographic image carrying vital sign information of asubject, the method comprising: obtaining image data of a skin region ofa subject in at least three different wavelength channels, wherein theimage data comprising two or more image frames acquired by detectinglight transmitted through or reflected from the skin region over time,wherein the image data comprise wavelength-dependent reflection ortransmission information in the at least three different wavelengthchannels, combining, per group of one or more pixels of the image framesand per time instant, image data values of the at least three differentwavelength channels to obtain a time-variant pulse signal per group,wherein the image data values of the at least three different wavelengthchannels are combined as a weighted combination of one of: temporallynormalized wavelength channels, and logarithm of wavelength channels,wherein the sum of the weights used for the combination is zero, andgenerating a photoplethysmographic image from a property of therespective pulse signals in a time window including at least two imageframes.
 15. A non-transitory computer readable medium comprising programcode that, when executed by a computer, causes the computer to; obtainimage data of a skin region of a subject in at least three differentwavelength channels, wherein the image data comprising two or more imageframes acquired by detecting light transmitted through or reflected fromthe skin region over time, wherein the image data comprisewavelength-dependent reflection or transmission information in the atleast three different wavelength channels, combine, per group of one ormore pixels of the image frame and per time instant, image data valuesof the at least three different wavelength channels to obtain atime-variant pulse signal per group, wherein the image data values ofthe at least three different wavelength channels are combined as aweighted combination of one of: temporally normalized wavelengthchannels, and logarithm of wavelength channels, wherein the sum of theweights used for the combination is zero, and generate aphotoplethysmographic image from a property of the respective pulsesignals in a time window including at least two image frames.